Reland of Adding backward compatibility header

Reland of Adding backward compatibility header (patchset #1 id:1 of https://codereview.webrtc.org/2908143003/ )

Reason for revert:
Take three of relanding this after all internal issues have been resolved.

Original issue's description:
> Revert of Adding backward compatibility header (patchset #1 id:1 of https://codereview.webrtc.org/2909923002/ )
> 
> Reason for revert:
> Breaks downstream project.
> 
> Original issue's description:
> > Adding backward compatibility header
> > 
> > This header will be removed ad soon as downstream projects will be
> > updated.
> > 
> > BUG=webrtc:4867
> > NOTRY=True
> > TBR=kjellander@webrtc.org
> > 
> > Review-Url: https://codereview.webrtc.org/2909923002
> > Cr-Commit-Position: refs/heads/master@{#18306}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/4fa8a97cc3268de73ad021ff56aaa9bf1a566ed4
> 
> TBR=kjellander@webrtc.org,magjed@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:4867
> 
> Review-Url: https://codereview.webrtc.org/2908143003
> Cr-Commit-Position: refs/heads/master@{#18308}
> Committed: https://chromium.googlesource.com/external/webrtc/+/6684fdce93ff25a837117cd7a811a6d86cbe4b20

TBR=kjellander@webrtc.org,magjed@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:4867
Review-Url: https://codereview.webrtc.org/2912773003
Cr-Commit-Position: refs/heads/master@{#18320}
Committed: https://chromium.googlesource.com/external/webrtc/+/d7620582342bb896b766bb306135b12aefbeb12c

[mbonadei, 2017-05-30 08:48:34]

Created Reland of Adding backward compatibility header

[mbonadei, 2017-05-30 08:50:21]

The CQ bit was checked by mbonadei@webrtc.org

[commit-bot: I haz the power, 2017-05-30 08:50:33]

CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.webrtc.org/...

[commit-bot: I haz the power, 2017-05-30 08:50:36]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1496134221847590, "parent_rev":
"2f08879fb19db40cc1c35c8233103006219db3fe", "commit_rev":
"d7620582342bb896b766bb306135b12aefbeb12c"}

[commit-bot: I haz the power, 2017-05-30 08:50:39]

Description was changed from

==========
Reland of Adding backward compatibility header (patchset #1 id:1 of
https://codereview.webrtc.org/2908143003/ )

Reason for revert:
Take three of relanding this after all internal issues have been resolved.

Original issue's description:
> Revert of Adding backward compatibility header (patchset #1 id:1 of
https://codereview.webrtc.org/2909923002/ )
> 
> Reason for revert:
> Breaks downstream project.
> 
> Original issue's description:
> > Adding backward compatibility header
> > 
> > This header will be removed ad soon as downstream projects will be
> > updated.
> > 
> > BUG=webrtc:4867
> > NOTRY=True
> > TBR=kjellander@webrtc.org
> > 
> > Review-Url: https://codereview.webrtc.org/2909923002
> > Cr-Commit-Position: refs/heads/master@{#18306}
> > Committed:
https://chromium.googlesource.com/external/webrtc/+/4fa8a97cc3268de73ad021ff5...
> 
> TBR=kjellander@webrtc.org,magjed@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:4867
> 
> Review-Url: https://codereview.webrtc.org/2908143003
> Cr-Commit-Position: refs/heads/master@{#18308}
> Committed:
https://chromium.googlesource.com/external/webrtc/+/6684fdce93ff25a837117cd7a...

TBR=kjellander@webrtc.org,magjed@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:4867
==========

to

==========
Reland of Adding backward compatibility header (patchset #1 id:1 of
https://codereview.webrtc.org/2908143003/ )

Reason for revert:
Take three of relanding this after all internal issues have been resolved.

Original issue's description:
> Revert of Adding backward compatibility header (patchset #1 id:1 of
https://codereview.webrtc.org/2909923002/ )
> 
> Reason for revert:
> Breaks downstream project.
> 
> Original issue's description:
> > Adding backward compatibility header
> > 
> > This header will be removed ad soon as downstream projects will be
> > updated.
> > 
> > BUG=webrtc:4867
> > NOTRY=True
> > TBR=kjellander@webrtc.org
> > 
> > Review-Url: https://codereview.webrtc.org/2909923002
> > Cr-Commit-Position: refs/heads/master@{#18306}
> > Committed:
https://chromium.googlesource.com/external/webrtc/+/4fa8a97cc3268de73ad021ff5...
> 
> TBR=kjellander@webrtc.org,magjed@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:4867
> 
> Review-Url: https://codereview.webrtc.org/2908143003
> Cr-Commit-Position: refs/heads/master@{#18308}
> Committed:
https://chromium.googlesource.com/external/webrtc/+/6684fdce93ff25a837117cd7a...

TBR=kjellander@webrtc.org,magjed@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:4867

Review-Url: https://codereview.webrtc.org/2912773003
Cr-Commit-Position: refs/heads/master@{#18320}
Committed:
https://chromium.googlesource.com/external/webrtc/+/d7620582342bb896b766bb306...
==========

[commit-bot: I haz the power, 2017-05-30 08:50:39]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/external/webrtc/+/d7620582342bb896b766bb306...